Question: What is that final thump on the takeoff roll that the plane's undercarriage makes just as the plane goes airborne?

-- submitted by reader Larry D Owens, Atlanta

Answer: You are probably referring to the main landing gear struts extending to their full travel. It is common to hear a "thump" caused by this.

Q: Captain Cox, on CRJ aircraft approximately 30-60 seconds before touchdown, I often hear three short beeps from the direction of the cockpit. Do you know what those beeps indicate? These are not the same as the seatbelt or other cabin-crew signals. Thanks!

-- Dan Parker - Madison, Wis.

A: It is possible that the noise you are hearing is the autopilot being disconnected. Modern airplanes have an aural warning when the autopilot disconnects or is manually disconnected. This notifies the pilot that he or she is now responsible to manually fly the airplane.

During the approach to landing, it is normal for the pilots to disconnect the autopilot. When they do, passengers can hear the disconnect warning and around a minute from touchdown would not be an uncommon time.

Q: Occasionally, usually during taxi, I hear a mechanical sound as if something is twisting or rotating, similar to the pneumatic wrenches you hear in auto repair shops. What is this noise?

-- Blake, Louisiana

A: I suspect you are referring to the Airbus A320 family when the Power Transfer Unit (PTU) activates during taxi in. The PTU has been described as a "dog barking," "a NASCAR tire lug gun" or "sawing."

The noise is the normal sound the PTU makes when it powers one of the hydraulic systems. During taxi in, one engine is often shut down causing the PTU to power the opposite hydraulic system.

Q: What is the intermittent squeaking noise that sometimes airplanes make while taxiing? I once heard it's caused when the taxiing takes place under the thrust of only one engine for fuel saving. Is that true?

-- submitted by reader Ignacio, New York City

A: You pose a difficult question because "squeaking noises" can come from several sources. There are times when the interior panels squeak when there is a bump in a taxiway, a nose gear strut can squeak occasionally, or there can be other causes.

Taxiing on one engine does not cause an airplane to squeak. Of course, you may be using "squeak" to describe the same sound as the previous questioner, in which case, see above.